interpreted as violet-blue, green-yellow, and red,
and register the intensity of light. The three-cone human eye system is called trichromacy.
Most mammals are dichromic, some creatures
including pigeons are pentachromic, while
some species of dragonfly have up to 30 types of
receptor including ultraviolet sensors. Around
one in eight women is tetrachromic. This is due
to a mutation of the genes in the X-chromosone
for the light-absorbing opsin molecules in the
M and L cones. It ought to mean that they can
differentiate up to 100 times as many colours as
the rest of us within the same 700nm to 390nm
range. Sadly, only a fraction actually perceive
more than usual, which indicates an issue with
the brain, not the eye.

According to a report on hospital lighting forBritain’s National Health Service only four percent of registered blind people in the UK have nosight at all and most are able to make out shapesand contrast in colour. As the light dims, though,there is a transition between photopic (cone-based) and scotopic (rod cell-based) night visionduring which we first perceive a shift in colourcontrast, known as the Purkinje effect, and then,essentially, we all become colour blind. Needlessto say, objects do not change colour in the dark,only our perception changes. Throughout, athird type of photoreceptor sends the brain non-visual information affecting the body clock.In 1931, the Commission Internationale del’Eclairage or International Lighting Commission(CIE) agreed a means of quantifying all thecolours that an ‘average’ trichromic personcan see. With only minor refinements the CIE1931 Colour Space developed by William DavidWright and John Guild remains in use today. Ituses combinations of all the colours that can bemixed from the three primaries absorbed by theeye – red, green and blue – to map colours onto a sail-like volume called the gamut, a termborrowed from music where it refers to a rangeof available notes.The brain has to carry out a kind of statistical